The dangers of obstructed breathing during sleep are well known in relation to the Obstructive Sleep Apnea (OSA) syndrome. Apnea, hypopnea and heavy snoring are recognized as causes of sleep disruption and risk factors in certain types of heart disease.
The monitoring of upper airway pressure-flow relationships in obstructive sleep apnea has been described in Smith et al., 1988, J. Appl. Physiol. 64: 789-795. FIG. 1 of that article shows polygraphic sleep recordings at varying levels of increasing nasal pressure. It was noted that inspiratory volumetric flow plateaued in certain breaths suggesting the presence of airflow limitation. Pressure-flow curves were constructed by plotting midinspiratory airflow against either mask pressure or endoesophageal pressure. The pressure-flow plots of nasal pressure against mean midinspiratory flow were then fit by least-squares linear regression to calculate resistance upstream to the collapsible site.
The effect of positive nasal pressure on upper airway pressure-flow relationships has been described in Schwartz et al., 1989, J. Appl Physiol. 66: 1626-1634. FIG. 4 of the article shows that pressure-flow tracings plateau at a low pressure level. It was further shown when the pressure was increased, flow did not plateau.
The common method of treatment of these syndromes is to administer Continuous Positive Airway Pressure (CPAP). The procedure for administering CPAP treatment has been documented in both the technical and patent literature. Briefly stated, CPAP treatment acts as a pneumatic splint of the airway by the provision of a positive pressure, usually in the range 4-20 cm H2O. The air is supplied by a motor driven blower whose output passes via an air delivery device to sealingly engage a patient's airway. A mask, tracheotomy tube, endotracheal tube, nasal pillows or other appropriate device may be used. An exhaust port is provided in a delivery tube proximate to the air delivery device. Other forms of CPAP, such as bi-level CPAP, and self-titrating CPAP, are described in U.S. Pat. Nos. 5,148,802 and 5,245,995 respectively.
With regard to the control of CPAP treatment, various techniques are known for sensing and detecting abnormal breathing patterns indicative of obstruction. For example, U.S. Pat. No. 5,245,995 describes how snoring and abnormal breathing patterns can be detected by inspiration and expiration pressure measurements while sleeping, thereby leading to early indication of preobstructive episodes or other forms of breathing disorder. Particularly, patterns of respiratory parameters are monitored, and CPAP pressure is raised on the detection of pre-defined patterns to provide increased airway pressure to ideally prevent the occurrence of the obstructive episodes and the other forms of breathing disorder.
Similarly, U.S. Pat. No. 5,335,654 (Rapoport) lists several indices said to be indications of flow limitation and/or partial obstruction patterns including: (1) The derivative of the flow signal equals zero; (2) The second derivative between peaks of the flow signal is zero for a prolonged interval; (3) The ratio of early inspirational flow to midinspirational flow is less than or equal to 1. The patent further lists events said to be indications of obstructions: (1) Reduced slope of the line connecting the peak inspiratory flow to the peak expiratory flow; (2) Steep upward or downward stroke (dV/dt) of the flow signal; and (3) Ratio of inspiratory flow to expiratory flow over 0.5.
U.S. Pat. No. 5,645,053 (Remmers) describes calculating a flatness index, wherein flatness is defined to be the relative deviation of the observed airflow from the mean airflow. In Remmers, individual values of airflow are obtained between 40% and 80% of the inspiratory period. The mean value is calculated and subtracted from individual values of inspiratory flow. The individual differences are squared and divided by the total number of observations minus one. The square root of this result is used to determine a relative variation. The relative variation is divided by the mean inspiratory airflow to give a relative deviation or a coefficient of variation for that breath.
In commonly owned U.S. Pat. No. 5,704,345, Berthon-Jones also discloses a method for detecting partial obstruction of a patient's airway. Generally, the method involves a determination of two alternative obstruction index values based upon the patient's monitored respiratory airflow. Either obstruction index may then be compared to a threshold value. Essentially, the index values may be characterized as shape factors that detect a flattening of an inspiratory portion of a patient's respiratory airflow. The first shape factor involves a ratio of the mean of a midportion of the inspiratory airflow of the breathing cycle and the mean of the inspiratory airflow. The formula for shape factor 1 is as follows:
      shapefactor_    ⁢    1    =                    1        33            ⁢                        ∑                      t            =            16                    48                ⁢                              f            s                    ⁡                      (            t            )                                M  where fs(t) is a sample of the patient's inspiratory airflow and M is the mean of inspiratory airflow given by the following:
  M  =            1      65        ⁢                  ∑                  t          =          1                65            ⁢                        f          s                ⁡                  (          t          )                    A second shape factor involves a ratio of the Root Mean Square deviation of a midportion of inspiratory airflow and the mean inspiratory airflow according to the formula:
      shapefactor_    ⁢    2    =                              1          33                ⁢                              ∑                          t              =              16                        48                    ⁢                                    (                                                                    f                    s                                    ⁡                                      (                    t                    )                                                  -                M                            )                        2                                M  Berthon-Jones further discloses a scaling procedure applied to the inspiratory airflow samples such that the mean M of the samples fs(t) is unity (M=1). This scaling procedure simplifies both shape factor formulas. Additional adjustments to fs(t) including averaging and the elimination of samples from erratic breaths such as coughs, sighs, hiccups, etc., are also taught by Berthon-Jones. The foregoing U.S. patent is hereby incorporated by reference.
The present invention involves an improved method and apparatus for detecting some forms of obstruction based upon the flattening of the inspiratory airflow.